Accompanying recent popularization of computers, an ink jet printer is widely used for printing letters or an image on paper, film, cloth or the like not only at offices but also at homes.
The ink jet recording method includes a system of jetting out a liquid droplet by applying a pressure using a piezo element, a system of jetting out a liquid droplet by generating bubbles in an ink under heat, a system of using an ultrasonic wave, and a system of jetting out a liquid droplet by suction using an electrostatic force. The ink used for such ink jet recording includes an aqueous ink, an oily ink and a solid (fusion-type) ink. Among these inks, the aqueous ink is predominating in view of production, handleability, odor, safety and the like.
The coloring agent used in such an ink for ink jet recording is required to have high solubility in a solvent, enable high-density recording, provide good color, exhibit excellent fastness to light, heat, air, ozone, water and chemicals, ensure good fixing property to an image-receiving material and less blurring, give an ink having excellent storability, have high purity and no toxicity, and be available at a low cost. However, it is very difficult to find out a coloring agent satisfying these requirements in a high level. In particular, a coloring agent having good magenta or cyan color and excellent fastness to light and ozone is strongly demanded.
Various dyes and pigments for ink jet recording have been already proposed and actually used, however, a coloring agent satisfying all requirements is not yet found out at present. Conventionally well-known dyes and pigments having a color index (C.I.) number can hardly satisfy both color and fastness required of the ink for ink jet recording.
On the other hand, in the case of printing a high-quality image such as photograph by the ink jet recording method, when the hitting volume is increased, the image can be printed at a high speed without impairing the reproducibility of high density, however, this has a problem that the image is poor in the smoothness, that is, the roughness is outstanding. The roughness of image is reduced by decreasing the hitting volume, however, this is accompanied with a problem that the high-speed printing with good reproducibility of high density cannot be attained.
Furthermore, in the case of printing a high-quality image such as photograph in a high productivity by the ink jet recording, the image must be recorded on a recording paper in a high density and at the same time, printed at a high speed. However, with an ink in a low concentration, a large number of ink droplets must be hit and if the ink is hit at a low frequency, the recording time is prolonged and the productivity disadvantageously decreases.
For solving this problem, a method of hitting the ink using a printer head having a large number of nozzles is proposed, however, due to the limitation of the driving electric power or effect of the heat generated in the printer head on the image, the number of head nozzles which can be simultaneously driven is limited. Actually, in the case of a type where the recording is performed by moving a recording material and a printer head in the directions orthogonal to each other, the number of printer heads which can be simultaneously driven is from a few tens to about 200 and even in the case of a type called line head where the head is fixed, a few hundreds. A printer head capable of recording an image in a high density at a high speed is being demanded.
On recording an image using the above-described ink, the hitting speed at hitting an ink from a nozzle seriously affects the quality of image recorded. If the hitting speed is low, the positions of ink droplets landed on the recording material disperse and this causes problems. That is, in the case of recording an image with one color, indentation called jaggy is generated in the line part and in the case of recording a gradation image using multiple colors, the recorded image may be unevenly tinged or the image may lack in the smoothness and may be roughened.
When the hitting speed is high, these problems scarcely occur but in the case of a low-concentration ink, a large number of ink droplets must be hit to give a predetermined density and therefore, even if the hitting speed is high, the recording disadvantageously takes a long time as compared with a high-concentration ink.